2.3.csm: fix mac os issues

1. Lower OpenGL demands: the demo works fine with v4.1. No need to require v4.6 which is not available on mac os.

2. Use actual framebuffer size to fix viewport issues on retina displays.

src/8.guest/2021/2.csm/10.debug_cascade.vs

@@ -1,4 +1,4 @@
-#version 460 core
+#version 410 core
 layout (location = 0) in vec3 aPos;
 
 uniform mat4 view;
@@ -7,4 +7,4 @@ uniform mat4 projection;
 void main()
 {
     gl_Position = projection * view * vec4(aPos, 1.0);
-}
+}